Time-dependent drift degradation due to the progressive failure of rock bridges along discontinuities

An important element of time-dependent drift degradation is the progressive failure of intact segments along discontinuities, referred to as rock bridges. A fracture mechanics model is developed to simulate the time-dependent failure of rock bridges along discontinuities. The time dependence of the rock bridge failure process is modeled utilizing subcritical crack growth. The rock bridges give an effective cohesion to the discontinuities, and this cohesion is time-dependent due to the time-dependent failure of the rock bridges. The resulting first-order differential equation for joint cohesion is implemented into the UDEC distinct element numerical code to model time-dependent drift degradation. The model and its implementation into UDEC are validated using several simple examples, including a direct shear test and a rigid block on a slope. Two time-dependent drift degradation examples are then shown, one with and one without thermal loading. These examples used similar geometry, material parameters and in situ stresses as for the proposed underground drifts for the storage of nuclear waste at Yucca Mountain. Both with and without thermal loading, a large zone develops around the excavation where the joint cohesion and tensile strength drop to zero due to the failure of rock bridges. This in turn results in an excavation that is significantly less stable than if time dependence was not included. The results demonstrate the importance of time-dependence on the stability of underground excavations in hard rock.